1. Technical Field of the Invention
This invention relates to pressure vessels used in process operations requiring extreme cleanliness and operated at elevated pressures and temperatures, and in particular to pressure vessel design and closure mechanisms that facilitate easier and cleaner loading and closing of pressure vessels used in automated wafer treatment processes in a production environment.
2. Background Art
There is a general requirement in the semiconductor industry, and in other industries as well, for conducting processes that require enclosures or pressure vessels that can be loaded with wafers or other objects to be processed, permit the admittance and removal of process fluids or materials necessary to the process after the enclosure is sealed, and be elevated and ranged in pressure and temperature to, in some cases, extremes. Some processes are much more critical as to contamination, and require quick and close control of temperature, pressure, and the volume and timing of the introduction of process fluids to the pressure vessel. Add to that the demand for conducting these processes in a production mode, and the growing sophistication of the processes themselves, and it is amply clear that improvements in pressure vessels are needed.
This disclosure relates in particular to pressure vessels used in operations requiring extreme cleanliness and operated at elevated or high pressures up but not limited to 10,000 psi (pounds per square inch), and further, to pressure vessel design and isolated lid locking mechanisms that facilitate easier loading and locking of pressure vessels used in automated wafer treatment processes in a production environment. By way of example of the typical requirements placed on a pressure vessel, one process is described below.
This example refers to the manufacture of MEMS (Micro Electro Mechanical Systems) devices where the process agent is carbon dioxide in liquid and supercritical form. Other semiconductor related applications with strict cleanliness requirements, such as photoresist stripping, wafer cleaning, particulate removal, dry resist developing, and material deposition, all suffer from the same pressure vessel deficiencies, which include particle generation upon closing, closure mechanisms that are not suited for, quick closing, problems with automatically loading and unloading the vessel, and problems with the integration of the apparatus in a production line.
One method of manufacturing Microetectromechanical systems (MEMS) based devices is Sacrificial Surface Micromachining (SSM) or Surface Micromachining. In a simple xe2x80x9canchoredxe2x80x9d SSM silicon based production process, there is deposited on a substrate such as Silicon, a sacrificial layer of material such as grown SiO2, silicon dioxide, or some type of photoresist material in the case of processes for stripping photoresist. The sacrificial material is etched to open a hole for the anchor of the structure. A structural material such as polysilicon, or metal, is then deposited on the sacrificial material. Finally, the sacrificial material is etched away to release the structural layer, creating the microstructure. These steps can be repeated to form more complex multilevel structures.
After the removal of a sacrificial layer the substrate is rinsed. Upon evaporation of the rinsing liquid that is trapped between the xe2x80x9creleasedxe2x80x9d structure and the substrate surface a capillary force is generated that pulls down the released structure until it touches the substrate surface. The surface tension of the rinsing liquid generates the capillary force upon evaporation due to liquid/vapor phase transition. Stiction or adhesion occurs when the released structure adheres to the other surface, as for example in the case where a polysilicon or metal cantilevered beam adheres to the substrate, resulting in a defect in the device.
In a laboratory method, originally developed by researchers of the University of California at Berkeley, a silicon wafer containing a pattern of microelectromechanical structures, having been fabricated in the conventional manner, is arranged in a pressure vessel, submerged in methanol. The pressure vessel is first filled with methanol, and the wafer quickly transferred into the vessel, being maintained underneath a liquid layer of the methanol during the transport and loading process. The vessel is sealed, and a through-flow of liquid carbon dioxide introduced for about 15 minutes, during which time the methanol is rapidly absorbed into the liquid carbon dioxide and carried out of the pressure vessel.
When the vessel cavity has been entirely purged of methanol and is completely filled with pure liquid carbon dioxide, heat is applied uniformly for several minutes, causing the carbon dioxide to transition to its supercritical phase at a temperature higher than 31.1 degrees centigrade and a pressure of higher than 1073 psi, in which it has no surface tension. For drying the microstructured substrate the vessel cavity is vented from the supercritical state to atmosphere pressure while keeping the temperature higher than the critical temperature of the carbon dioxide. A phase transition does not occur, hence a capillary force is not generated and stiction is avoided completely. It is at this point that the benefit of the process is realized, as no liquid/vapor interface occurs during this transition to cause unwanted surface tension.
Other processes like photoresist stripping or wafer cleaning in general, which use a process agent in gaseous, liquid and supercritical form, are similar to the MEMS drying process in so far as they also apply the process agent to the vessel in a similar way and are finalized by the drying step. The MEMS process therefore is considered as exemplary for all applications with gaseous, liquid, and supercritical process agents where extreme cleanliness and high throughput are basic requirements.
There are several obvious problems with the laboratory set up that must be addressed in order to make the process sufficiently cost-effective and efficient for use in a production environment. The device is not suitable for integration into a production line with automated means for inserting and removing wafers; there is no safe transfer mechanism to ensure that a liquid layer is maintained on the wafer during the transport or transfer process; the closing mechanism is manual and too slow; and the serially administered steps of the process are manually accomplished and too slow. The device is also lacking the safeguards required by industrial standards and regulations for production requirements.
In the production setups used currently, the pressure vessel is loaded by vertical placement through an open top port of the same or larger diameter of the wafers being processed, and is unloaded by reverse action. The vessel is typically closed by manually bolting or mechanically clamping the process vessel flanges and its cover flanges together around the perimeter to form a pressure seal. This apparatus and methodology is both slow and prone to introducing particulate contamination due to the mechanical interface and constant wearing of mating surfaces. The particulate is generated immediately within the loading and processing environment, and inevitably contaminates the materials being processed to some degree.
These contaminants are of particular concern in the semiconductor industry, as even trace amounts are sufficient to plague product quality and production efficiencies. When these perimeter flange latching mechanisms are semi-automated for faster closure or production purposes, the contamination problem is simply placed in a free-running mode that gets progressively worse if unattended.
There are many examples in prior art. One such example is an autoclave, quick opening door assembly as shown in prior art FIG. 1. It consists of a chamber flange, a rotating locking ring and the door flange. The door and vessel are clamped. and unclamped by the rotation of the locking ring only. As the ring rotates, surfaces of the mating wedges force the chamber flange tight against the gasket providing a leak proof static seal. Due to the contact of the wedges sliding across each other, particles are generated that eventually contaminate the process beyond acceptable tolerances.
A further problem with traditional pressure vessels in a production environment is the difficulty in adapting them to the standard wafer handling robots of the semiconductor industry. Complex carriage systems are often necessary for automation of the loading and extracting of materials being processed, involving complex transitions between horizontal and vertical transport of the wafers between processing stations.
The invention is fundamentally a pressure vessel system for conducting industrial processes, suitable for applications where extremes of cleanliness, temperature, and pressure are encountered, and adaptation to automated production systems is important. In its simplest form, it is a stationary pressure vessel with connections or ports through which process materials or ingredients are delivered and removed, in combination with a vertically movable lid or cover. These two components are attached directly or indirectly to a common open framework for support. The closing and locking mechanisms are remote from the process environment of the pressure vessel and the space between the open cover and the pressure vessel. The apparatus is adaptable to processes requiring pressures up to the 5,000-10,000 psi (pounds per square inch) pressure range and beyond by proper design. Temperature of the contents can be ranged as the process requires, when configured with suitable internal and or external heating sources.
The apparatus is adaptable to a horizontal, pass-through conveyor system, any standard wafer handling robot systems, or any other handling system for delivering and loading articles to be processed under pressure, onto the cover. The vertically operated cover can carry a wafer cassette, a single wafer, or other object being processed into the pressure vessel for processing, and out again for pickup and further transport. A lift and lock mechanism for operating the cover is fully contained on the backside or outside of the cover, and shielded so as to isolate any particulate generated by the lift and lock mechanism from the loading and processing environment.
It is an object of the invention to provide a pressure vessel system with less risk of contamination from closing and locking mechanisms.
Is a further object to provide a pressure vessel system readily adaptable to horizontally oriented wafer transfer or handling systems, where the vertical component of placement within the pressure vessel is accomplished or supported by the pressure vessel and lid closing system.
The invention extends to a system consisting of an open support frame, an inverted pressure vessel, a vertically movable underside cover, and a lift and lock mechanism, where the cover is movable by the lift and lock mechanism between a lower open position and an upper closed and locked position against the mouth of the inverted pressure vessel, and where the open support frame is configured with spaced apart uprights that permit bi-directional horizontal access to the top side of the cover when in the open position, and where the lift and lock mechanism is located below the cover and is mechanically shielded from direct exposure to the top side of the cover and the interior of the pressure vessel.
It is a further object of the invention to provide a pressure vessel system where the lift and lock mechanism has horizontal linear sliding blocks for locking the cover up tight against the mouth of the inverted pressure vessel.
It is a yet further object to provide a pressure vessel system where the lift and lock mechanism has a rotating lug lock system for locking the cover up tight against the mouth of the inverted pressure vessel.
Still other objectives and advantages of the present invention will become readily apparent to those skilled in this art from the detailed description, wherein we have shown and described a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by us on carrying out our invention.